The reaction proceeds via three separate chemical reactions:
phosphorylation of hydrogencarbonate to carboxyphosphate; a
nucleophilic attack of ammonia on carboxyphosphate yielding
carbamate; and the phosphorylation of carbamate forming carbamoyl
phosphate.

Two moles of ATP are utilized for the synthesis of one molecule of
carbamyl phosphate, making the reaction essentially irreversible.